1. Field of Invention
This invention relates to polypropylene foam sheets and a process for their manufacture. Specifically, this invention relates to polypropylene foam sheets which are rigid or semi-rigid and thermoformable into shaped articles for use in packaging and service applications.
2. Description of the Prior Art
A foamed plastic or cellular plastic has an apparent density which is decreased by the presence of numerous voids or cells dispersed throughout its mass (ASTM D883-80C). The cells may be interconnected (open-celled) and/or discrete and independent (closed-celled).
The prior art discloses various methods for the preparation of foamed plastics. These include leaching out a solid or liquid which is dispersed in a plastic, sintering small particles of a plastic and dispersing cellular particles in a plastic. However, the most widely used method involves the dispersion of a gaseous phase throughout a fluid polymer phase and the retention of the resultant expanded form.
The theory of the expansion process and the properties of various foamed plastics are reviewed in "Cellular Plastics", in Encyclopedia of Polymer Science and Engineering, vol. 3, pp. 1-59 (1985), which is incorporated herein by reference. As disclosed therein, the expansion process consists of three steps: creation of small discontinuities or cells in a fluid or plastic phase, growth of these cells to a desired volume and stabilization of the resultant cellular structure by physical or chemical means.
The formation of discontinuities or bubbles within the fluid polymer, may arise from gases that are injected into the fluid polymer, low boiling liquids that are incorporated into the system as blowing agents and volatilize due to increased temperature or decreased pressure, gases that are produced as a result of a chemical reaction within the fluid polymer and chemical blowing agents which undergo thermal decomposition to form a gas.
The rate of growth of the bubbles or cells depends upon the viscoelastic nature of the polymer phase, the blowing agent pressure, the external pressure on the foam, the cell size and the permeation rate of the blowing agent through the polymer phase.
Cell or bubble stabilization relates to cell wall stability and the drainage of material from the membrane or wall which separates cells. Increasing the viscosity of the fluid reduces the drainage effect. The viscosity increase may be caused by chemical reactions which increase molecular weight through polymerization or crosslinking, or by temperature reduction, ultimately below the second order transition or crystallization temperature to prevent polymer flow.
The present invention relates to rigid or semi-rigid foam sheets for use in food service applications. The prior art has utilized polystyrene for the manufacture of foam sheets for these applications. However, polystyrene articles suffer from low service temperature, and little or no photochemical or biological degradability and are relatively expensive.
Polypropylene does not have these undesirable characteristics. Various processes have been reported in the prior art for the preparation of flexible or rigid polypropylene foams. The processes are designed to promote the three-step process described hereinbefore, i.e. creation of cells in a fluid or plastic phase, growth of the cells and stabilization of the resultant cellular structure.
Blowing agents used in the preparation of polypropylene foam include azodicarbonamide (Lee et al, J. Appl. Polym. Sci. 32, 4639 (1986); EPO Pat. Appl. EP 190,021), chlorofluorocarbons (EPO Pat. Appl. EP 1791, EP 71,981, EP 181,637; U.K. Pat. 1,400,494; U.K. Pat. Appl. GB 2,099,434 A), carbon dioxide (EPO Pat. Appl. EP 291,764), hydrocarbons, e.g. propane, butane, pentane (U.K. Pat. 1,400,494; U.K. Pat. Appl. GB 2,099,434 A) and water (EPO Pat. Appl. EP 122,460).
Crystallization rate accelerators and/or nucleating agents used in the preparation of polypropylene foam include titanium dioxide (EPO Pat. Appl. EP 122,460; U.K. Pat. Appl. GB 2,099,434 A), talc (U.K. Pat. 1,400,494; U.K. Pat. Appl. GB 2,099,434 A), silica and silicates (EPO Pat. Appl. EP 1791; U.S. Pat. No. 4,467,052), zeolite 4A (EPO Pat. Appl. EP 178,282, EP 178,283), sodium benzoate (Colton, Plast. Eng. 44(8), 53 (1988) and dibenzylidene sorbitol (EPO Pat. Appl. EP 178,282).
Citric acid-sodium bicarbonate combinations are considered as blowing agents in some patents and as nucleating agents in other patents (EPO Pat. Appl. EP 178,283; U.K. Pat. 1,400,494; U.K. Pat. Appl. GB 2,099,434 A; U.S. Pat. No. 4,467,052).
The use of crosslinking agents during the preparation of a polypropylene foam has been reported in the prior art and include peroxides (Nojiri et al, Furukawa Review 2, 34 (1982) through Chem. Abstracts 97, 21725u (1982); EPO Pat. Appl. EP 181,637, 190,021) in the absence or presence of multifunctional vinyl monomers, azido functional silanes (EPO Pat. Appl. EP 181,637), vinyltrimethoxysilane (Lee et al, J. Appl. Polym. Sci. 32, 4639 (1986) and ionizing radiation in the presence of polyacrylic monomers (Nojiri et al, Furukawa Review 2, 34 (1982); U.S. Pat. No. 4,424,293).
Low density polypropylene foams "free from creases on the surface" have been prepared by incorporating high molecular weight fatty amides, amines or esters in the molten polyolefin (EPO Pat. Appl. EP 1791).
The prior art teaches that polypropylene is not a unique material, i.e. processes that are applicable to the preparation of foam or microcellular structures from other polymers are applicable to the preparation of polypropylene foams.
EPO Pat. Appl. EP 1791 describes "a process for the preparation of expanded thermoplastic synthetic resins" and discloses polyethylene, ethylene-vinyl acetate copolymer and isotactic polypropylene as the applicable thermoplastic resins.
EPO Pat. Appl. EP 71,981 describes "foamed polypropylene resin molded articles" and discloses the use of ethylene-propylene copolymer as well as polypropylene.
EPO Pat. Appl. EP 122,460 describes "resin foam produced by an aqueous medium" and discloses polymer foams from polypropylene, polyethylene and polystyrene.
EPO Pat. Appl. EP 291,764 describes the "extrusion of propylene polymer foam sheets" and discloses a process for extruding blends of ethylene-propylene block copolymers containing less than 20% ethylene with block copolymers containing less than 5% ethylene or random ethylene-propylene copolymers or polypropylene.
U.K. Pat. 1,400,494 describes "foamed polymeric sheet material and process therefor" and discloses polypropylene, high density polyethylene and nylon-12 as the preferred operable polymers while indicating that copolymers of ethylene with vinyl acetate or vinyl chloride can be conveniently used.
U.K. Pat. Appl. GB 2,099,434 A describes an "extrusion process for propylene resin foams" and states that the resin may be isotactic polypropylene, an ethylene-propylene block or random copolymer or blends of polypropylene with numerous olefin homopolymers and copolymers.
U.S. Pat. No. 3,637,458 describes "microcellular foam sheet" from a linear, thermoplastic crystalline polymer and claims isotactic polypropylene and linear polyethylene foam sheet.
U.S. Pat. No. 3,819,784 describes "a process for preparing molded polyolefin foam" and discloses that suitable polyolefins used in the process include low density polyethylene, high density polyethylene, isotactic polypropylene, poly-1-butene and copolymers of ethylene with propylene or vinyl acetate.
U.S. Pat. No. 3,830,900 describes "method of forming foamed plastic sheets" and discloses that the method is applicable to polyvinyl chloride, polystyrene, polyethylene, polypropylene and acrylonitrile-butadiene-styrene copolymers.
U.S. Pat. No. 4,467,052 describes a "tray for packaging food products" and discloses an injection molding process for the preparation of foam trays from blends of polypropylene and styrenebutadiene rubber.
Colton (Plast. Eng. 44(8), 53 (1988) describes "making microcellular foams from crystalline polymers" and discloses microcellular polypropylene and ethylene-propylene copolymer foams.
EPO Pat. Appl. 181,637 describes "lightly crosslinked linear olefinic polymer foams" prepared from melt blends of one or more polymers selected from high density polyethylene, linear low density polyethylene, polypropylene and polystyrene.
EPO Pat. Appl. EP 190,021 describes "heat-foamable crosslinked propylene resin compositions" and discloses blends of propylene-.alpha.-olefin copolymers or 1-butene-.alpha.-olefin copolymers with polypropylene.
U.S. Pat. No. 4,424,293 describes "crosslinkable polypropylene composition" and discloses foams from isotactic polypropylene and ethylene-propylene copolymer.
The prior art uses "polypropylene" as a self-explanatory term for a polymer prepared from propylene monomer. In some cases, the terms "isotactic polypropylene" and "crystalline polypropylene" are used. In only a few patents is the polypropylene characterized to any further extent.
EPO Pat. Appl. EP 71,981 discloses polypropylene foams prepared from resins having a latent heat of crystallization of 9-28 cal/g. U.S. Pat. No. 3,637,458 discloses polypropylene foams prepared from polymers of "at least film forming molecular weight, substantially free from crosslinking, and having a work-to-break (WTB) value of at least 10,000 inch-lbs/inch.sup.3 ". U.K. Pat. Appl. GB 2,099,434 A discloses polypropylene foams prepared from resins having a melt tension of at least 3 grams at 190.degree. C. and a maximum/minimum melt tension ratio of not more than 2.5/1.
Application of the processes of the prior art to generic or commercial polypropylene resins, described as polypropylene, isotactic polypropylene or crystalline polypropylene, fails to yield the polypropylene foam sheet of the present invention.